The present invention relates to a method for etching an organic film, a method for fabricating a semiconductor device and a pattern formation method.
For the purpose of increasing the operation speed and lowering the consumption power of semiconductor devices, decrease of the dielectric constant of an interlayer insulating film included in a multi-level interconnect structure is recently regarded as significant. In particular, an organic film with a small dielectric constant can be easily formed by spin coating and curing, and hence is regarded as a very promising interlayer insulating film of the next generation. A well known example of the organic film with a small dielectric constant is an organic film including an aromatic polymer as a base.
In order to fabricate a device with a refined design rule of a gate length of 0.18 xcexcm or less, a fine interconnect processing technique of approximately 0.25 xcexcm or less is necessary, and the design rule is considered to be more and more refined in the future. An organic film is generally patterned by plasma etching, but a fine pattern of 0.25 xcexcm or less is very difficult to form from an organic film.
Known examples of the plasma etching employed for an organic film are a process using an etching gas including a N2 gas and a H2 gas as principal constituents (reported by M. Fukusawa, T. Hasegawa, S. Hirano and S. Kadomura in xe2x80x9cProc. Symp. Dry Processxe2x80x9d, p. 175 (1998)) and a process using an etching gas including a NH3 gas as a principal constituent (reported by M. Fukusawa, T. Tatsumi, T. Hasegawa, S. Hirano, K. Miyata and S. Kadomura in xe2x80x9cProc. Symp. Dry Processxe2x80x9d, p. 221 (1999).
One of conventional etching methods will now be described as Conventional Example 1 referring to the result obtained by etching an organic film with a magnetic neutral loop discharge (NLD) plasma etching system manufactured by Ulvac Corporation (xe2x80x9cSiO2 Etching in magnetic neutral loop discharge plasmaxe2x80x9d, W. Chen, M. Itoh, T. Hayashi and T. Uchida, J. Vac. Sci. Technol., A16 (1998) 1594).
In Conventional Example 1, an organic film is etched by using an etching gas including a N2 gas and a H2 gas as principal constituents. The present inventors have carried out the etching process of Conventional Example 1 under the following conditions:
Plasma etching system: NLD plasma etching system
Volume flow ratio per minute in standard condition of etching gas:
N2:H2=50 ml:50 ml
Antenna power:
1000 W (13.56 MHz)
Bias power: 200 W (2 MHz)
Pressure: 0.4 Pa
Substrate cooling temperature: 0xc2x0 C.
Etching time: 180 seconds
FIGS. 8A through 8D are cross-sectional SEM photographs of holes formed under the aforementioned etching conditions in organic films, and the holes of FIGS. 8A through 8D have diameters of 0.16 xcexcm, 0.18 xcexcm, 0.24 xcexcm and 0.40 xcexcm, respectively. In FIGS. 8A through 8D, a reference numeral 101 denotes a silicon substrate, a reference numeral 102 denotes an organic film to be etched, and a reference numeral 103 denotes a mask pattern of a silicon oxide film used as a mask in etching the organic film 102. The organic film 102 has a thickness of approximately 1.02 xcexcm, and the mask pattern 103 has a thickness of approximately 240 nm.
In a multi-level interconnect structure of a semiconductor device, a lower interconnect, an interlayer insulating film and an upper interconnect are successively stacked, and the lower interconnect and the upper interconnect are connected to each other through a pillar-shaped plug formed in the interlayer insulating film. Also, single damascene and dual damascene methods have recently been developed. In the single damascene method, a via hole or an interconnect groove is formed in an interlayer insulating film and is subsequently filled with a metal material, so as to form a connection plug or a metal interconnect. In the dual damascene method, a via hole and an interconnect groove are formed in an interlayer insulating film and are subsequently filled with a metal material, so as to simultaneously form a connection plug and a metal interconnect.
Now, the conventional single damascene method will be described as Conventional Example 2 with reference to FIGS. 9A through 9E and 10A through 10D.
First, as is shown in FIG. 9A, a laminated metal interconnect consisting of a first barrier metal layer 112, a metal film 113 and a second barrier metal layer 114 is formed on a semiconductor substrate 111. Then, as is shown in FIG. 9B, an organic film 115 is formed on the metal interconnect, and thereafter, a silicon oxide film 116 is formed on the organic film 115 as is shown in FIG. 9C.
Then, a resist pattern 117 is formed on the silicon oxide film 116 by a known lithography technique as is shown in FIG. 9D. Thereafter, the silicon oxide film 116 is subjected to plasma etching (dry etching) using the resist pattern 117 as a mask, so as to form a mask pattern 116A from the silicon oxide film 116 as is shown in FIG. 9E.
Next, the organic film 115 is etched by the method for Conventional Example 1 by using the mask pattern 116A, so as to form a recess 118 for a via hole or an interconnect groove in the organic film 115 as is shown in FIG. 10A. Since the resist pattern 117 is formed from an organic compound, it is removed during the etching of the organic film 115.
Subsequently, a third barrier metal layer 119 of TiN or TaN with a small thickness is formed on the wall of the recess 118 by sputtering as is shown in FIG. 10B.
Then, the recess 118 is filled with a metal material film 122 by chemical vapor deposition (CVD) or plating as is shown in FIG. 10C, and a portion of the metal material film 122 formed outside the recess 118 is removed by chemical mechanical polishing (CMP). Thus, a connection plug or metal interconnect 123 is formed as is shown in FIG. 10D.
The conventional dual damascene method will now be described as Conventional Example 3 with reference to FIGS. 11A through 11D, 12A through 12C and 13A through 13C.
First, as is shown in FIG. 11A, a lower laminated metal interconnect consisting of a first barrier metal layer 132, a metal film 133 and a second-barrier metal layer 134 is formed on a semiconductor substrate 131. Then, a first organic film 135 is formed on the lower metal interconnect as is shown in FIG. 11B, and a first silicon oxide film 136 is formed on the first organic film 135 as is shown in FIG. 11C.
Next, a first resist pattern 137 having an opening for a via hole is formed on the first silicon oxide film 136 by a known lithography technique as is shown in FIG. 11D. Then, the first silicon oxide film 136 is subjected to plasma etching (dry etching) by using the first resist pattern 137 as a mask, so as to form a first mask pattern 136A from the first silicon oxide film 136 and remove the first resist pattern 137 as is shown in FIG. 12A. Thereafter, a top face of the first mask pattern 136A is cleaned so as not to damage the first organic film 135.
Then, as is shown in FIG. 123, a second organic film 138 is formed on the first mask pattern 136A, and a second silicon oxide film 139 is formed on the second organic film 138.
Next, as is shown in FIG. 12C, a second resist pattern 140 with an opening for an interconnect groove is formed on the second silicon oxide film 139. Thereafter, the second silicon oxide film 139 is etched by using the second resist pattern 140 as a mask, so as to form a second mask pattern 139A from the second silicon oxide film 139 as is:shown in FIG. 13A.
Subsequently, the second organic film 138 and the first organic film 135 are etched by the method for Conventional Example 1, so as to form an interconnect groove 141 by transferring the second mask pattern 139A onto the second organic film 138 and form a via hole 142 by transferring the first mask pattern 136A onto the first organic film 135 as is shown in FIG. 13B. FIG. 13B shows a state where the via hole 142 is being formed in the first organic film 135, and as shown in this drawing, a deposition including a reaction product generated through the reaction between the etching gas and the first organic film 135 and silicon released from the first mask pattern 136A is adhered onto the wall of the via hole 142, resulting in forming a barrier wall 143 from the deposition.
Next, the second organic film 138 and the first organic film 135 are continuously etched by the method for Conventional Example 1, so as to completely form the via hole 142 in the first organic film 135 as is shown in FIG. 13C. Thereafter, the second barrier metal layer 134 is over-etched so as to completely remove the first organic film 135 remaining on the second barrier metal layer 134. The second resist pattern 140 is completely removed through the etching and the over-etching.
Then, although not shown in the drawings, a third barrier metal layer with a small thickness is formed on the walls of the interconnect groove 141 and the via hole 142 in the same manner as in Conventional Example 2, and the interconnect groove 141 and the via hole 142 are filled with a metal material film. Thereafter, a portion of the metal material film formed outside the connection groove 141 is removed by the CMP. Thus, a connection plug and an upper metal interconnect are formed.
As methods of forming a mask pattern through dry development (plasma etching) of an organic film, a top surface imaging (TSI) process, a three-layer resist process and the like are known.
In the top surface imaging process, a surface of an organic film resulting from pattern exposure is subjected to silylation, so as to selectively form a silylated layer on an exposed or unexposed portion of the organic film. Then, the organic film is subjected to dry development (plasma etching) using the silylated layer as a mask, so as to form a resist pattern.
Now, a pattern formation method using dry development (top surface imaging process) will be described as Conventional Example 4 with reference to FIGS. 14A through 14D.
First, as is shown in FIG. 14A, an organic film 152 is formed on a semiconductor substrate 151, and then a silylation target layer 153 is formed on the organic film 152.
Next, as is shown in FIG. 14B, the silylation target layer 153 is irradiated with exposing light 154 through a photomask 155 for selectively allowing the light to pass, so as to selectively form a decomposed layer 156 in the silylation target layer 153.
Then, as is shown in FIG. 14C, with the substrate temperature increased, a gaseous silylation reagent 157 is supplied onto the silylation target layer 153, so as to selectively silylate a non-decomposed portion (a portion excluding the decomposed layer 156) of the silylation target layer 153. Thus, a silylated layer 158 is formed. Instead of silylating the non-decomposed portion, the decomposed layer 156 may be silylated to form the silylated layer 158.
Next, the organic film 152 is etched by the method for Conventional Example 1 by using the silylated layer 158 as a mask, so as to form an organic film pattern (mask pattern) 152A from the organic film 152 as is shown in FIG. 14D.
Another pattern formation method using dry development (three-layer resist process) will now be described as Conventional Example 5.
First, an organic film and a silicon oxide film are successively formed on a semiconductor substrate, and a thin resist pattern is then formed on the silicon oxide film.
Then, the silicon oxide film is subjected to plasma etching by using the resist pattern as a mask, so as to form a mask pattern by transferring the resist pattern onto the silicon oxide film. Thereafter, the organic film is subjected to dry development by using the mask pattern, so as to form a fine organic film pattern with a high aspect ratio from the organic film.
Next, by using a two-layer mask pattern consisting of the mask pattern and the organic film pattern, an etch target film formed on the semiconductor substrate is etched. In this manner, a fine pattern that cannot be resolved by using a single layer resist can be formed in the etch target film.
The present inventors have carried out the etching method for an organic film of Conventional Example 5 by using an etching gas including an 02 gas under the following etching conditions:
Plasma etching system: NLD plasma etching system
Flow rate per minute in standard condition of etching gas: O2=90 ml
Antenna power: 1000 W (13.56 MHz)
Bias power: 400 W (2 MHz)
Pressure: 0.133 Pa
Substrate cooling temperature: 0xc2x0 C.
Etching time: 4 minutes
FIGS. 15A and 15B are cross-sectional SEM photographs of holes formed in organic film patterns by the pattern formation method for Conventional Example 5, and the holes of FIGS. 15A and 15B have diameters of 0.18 xcexcm and 0.4 xcexcm, respectively. In FIGS. 15A and 15B, a reference numeral 171 denotes a silicon substrate, a reference numeral 172 denotes an organic film pattern, and a reference numeral 173 denotes a mask pattern of a silicon oxide film. The resist pattern formed on the mask pattern 173 is eliminated during formation of the organic film pattern by the dry development, and hence, the etch target film deposited on the silicon substrate 171 is etched by using a two-layer mask pattern consisting of the organic film pattern 172 and the mask pattern 173.
The etch shape (the cross-sectional shape of the hole) is apparently a good anisotropic shape (vertical shape) as is shown in FIGS. 8A through 8D.
It is, however, understood through detailed observation of FIGS. 8A through 8D that the hole actually has a bowing cross-section. A bowing cross-section means an arched overhang cross-section. As is obvious from FIGS. 8A through 8D, the hole formed in the organic film 102 through the etching has a larger diameter than the diameter of the opening of the mask pattern 103.
Accordingly, the etching method for Conventional Example 1 has a problem that a hole formed in the organic film 102 cannot have a cross-section tapered toward the bottom (hereinafter referred to as a forward taper cross-section).
In Conventional Example 2, when the organic film 115 is etched by the method for Conventional Example 1, the recess 118 is formed to have a bowing cross-section as is shown in FIG. 10A.
Since the cross-section of the recess 118 is in the bowing shape, when the third barrier metal layer 119 with a small thickness is formed on the wall of the recess 118, the third barrier metal layer 119 cannot be uniformly formed on the wall of the recess 118 as is shown in FIG. 10B. Specifically, the third barrier metal layer 119 is separated (disconnected) in a portion 120 just below the mask pattern 116A on the wall of the recess 118 and on a bottom 121 of the recess 118.
Accordingly, in forming the connection plug or the metal interconnect 123 by filling the recess 118 with the metal material film 122 by the CVD or plating, the metal material film 122 cannot be uniformly filled. Specifically, since the third barrier metal layer 119 is separated in the portion 120 just below the mask pattern 116A on the wall of the recess 118 and on the bottom 121 of the recess 118, the third barrier layer 119 is electrically insulated, namely, separated. Therefore, for example, when the metal material film 122 of copper is filled by electro-plating, a potential cannot be applied to a portion of the third barrier metal layer 119 inside the recess 118, and hence, the metal material film 122 cannot be uniformly filled in the recess 118. Alternatively, when the recess 118 is filled with the metal material film 122 of tungsten, a tungsten film is abnormally grown in the separated portions of the third barrier metal layer 119, and hence, the metal material film 122 cannot be uniformly filled in the recess 118. Since the metal material film 122 cannot be uniformly filled in the recess 118 in this manner, the connection plug or metal interconnect 123 is defective. As a result, the electric characteristic is disadvantageously degraded so as to degrade the reliability of the semiconductor device.
In Conventional Example 3, when the second organic film 138 and the first organic film 135 are etched by the method for Conventional Example 1, the interconnect groove 141 and the via hole 142 are formed to have a bowing cross-section as is shown in FIG. 13B.
Furthermore, the deposition including the reaction product and silicon is adhered onto the wall of the via hole 142 as described above. In addition, while the second barrier metal layer 134 is over-etched, the first mask pattern 136A serving as an effective etching mask for forming the via hole 142 is etched through ion sputtering during the etching. Accordingly, the opening of the first mask pattern 136A is enlarged as is shown in FIG. 13C. Therefore, the bowing cross-section of the via hole 142 in the first organic film 135 becomes more serious, and since the wall of the via hole 142 is thus recessed, the barrier wall 143 in a crown shape is formed on the bottom of the via hole 142 from the deposition including the reaction product generated in the etching and silicon.
Accordingly, when the interconnect groove 141 and the via hole 142 are filled with the metal material film by the CVD or plating to form the connection plug and the metal interconnect, the metal material film cannot be uniformly filled, and a connection failure is caused between the connection plug filled in the via hole 142 and the lower metal interconnect. As a result, a multi-level interconnect structure is difficult to form by the dual damascene method.
In Conventional Example 4, when the organic film 152 is etched by the method for Conventional Example 1, the opening 159 of the organic film pattern 152A is formed to have a bowing cross-section as is shown in FIG. 14D. When the organic film pattern 152A having such a hole with the bowing cross-section is used for etching, it is difficult to conduct highly precise etching.
In Conventional Example 5, since the organic film is subjected to the dry development carried out by plasma etching using an etching gas including an O2 gas as a principal constituent, the hole formed in the organic film pattern 172 has a larger diameter than the opening of the mask pattern 173 and the hole formed in the organic film pattern 172 has a bowing cross-section as is shown in FIGS. 15A and 15B. When the organic film pattern 172 having such a hole with the bowing cross-section is used for etching the etch target film, it is difficult to conduct highly precise etching.
Therefore, in a method proposed for suppressing the hole of the organic film pattern 172 from having a bowing cross-section, the dry development is carried out on the organic film with the actual substrate temperature kept at a temperature below the freezing point by setting the substrate cooling temperature (refrigerant temperature) to 20xc2x0 C. through 50xc2x0 C. below zero.
In order to attain such a low temperature, however, excessive cost and a large-scaled system are required, and hence, there arise problems of increase of the system cost and decrease of the system stability.
Accordingly, it is impossible to form a hole with a forward taper cross-section in an organic film pattern by the method for Conventional Example 5
Needless to say, the problems of Conventional Example 5 (the three-layer resist process) also arise in Conventional Example 4 (the top surface imaging process).
In consideration o the aforementioned conventional problems, a first object of the invention is forming a recess having a vertical or forward taper cross-section in an organic film by etching the organic film.
A second object of the invention is uniformly forming a barrier metal layer on the wall of a recess by forming the recess to have a vertical or forward taper cross-section in an organic film through plasma etching, so as to uniformly fill the recess with a metal material film.
A third object of the invention is forming an organic film pattern having an opening with a vertical or forward taper cross-section through dry etching (plasma etching) of an organic film, so as to realize precise etching with a large process margin.
In order to achieve the first object, the method for etching an organic film of this invention comprises a step of etching an organic film to be used as an interlayer insulating film by using plasma generated from an etching gas containing, as a principal constituent, a compound including carbon, hydrogen and nitrogen.
In the present method for etching an organic film, the plasma generated from the etching gas includes radicals of CHx (wherein x =1, 2 or 3) that can easily form a polymer on an etch target surface (the wall and the bottom of a recess), and the polymer of the CHx radicals adhered onto the wall of the recess formed in the organic film works as a sidewall protection film for inhibiting an ion assisted reaction. Accordingly, the recess can attain a vertical or forward taper cross-section.
In the method for etching an organic film, the compound is preferably methylamine, dimethylamine, trimethylamine, ethylamine or propylamine.
In this manner, the recess formed in the organic film can definitely attain a vertical or forward taper cross-section.
The method for fabricating a semiconductor device of this invention comprises the steps of forming an organic film on a semiconductor substrate; forming a mask pattern, on the organic film, including an inorganic compound as a principal constituent; and forming a recess in the organic film by selectively etching the organic film by using the mask pattern and by using plasma generated from an etching gas containing, as a principal constituent, a compound including carbon, hydrogen and nitrogen.
In the present method for fabricating a semiconductor device, the recess is formed in the organic film by the present method for etching an organic film. Therefore, a recess with a vertical or forward taper cross-section can be formed in the organic film, and the recess is prevented from having a bowing cross-section. Accordingly, a barrier layer can be uniformly formed on the wall of the recess without having a separated portion (disconnected portion), and hence, the recess can be definitely filled with a metal material film. As a result, a connection plug or a buried interconnect with a good electric characteristic can be formed.
In the method for fabricating a semiconductor device, the compound is preferably methylamine, dimethylamine, trimethylamine, ethylamine or propylamine.
In this manner, the recess formed in the organic film can definitely attain a vertical or forward taper cross-section.
In the method for fabricating a semiconductor device, the recess preferably includes a via hole and an interconnect groove formed above the via hole and is filled with a metal material film by a dual damascene method.
In this manner, a barrier layer can be uniformly formed on the walls of a via hole and an interconnect groove without having a separated portion (disconnected portion), and a crown-shaped barrier wall can be prevented from being formed on the bottom of the via hole. Therefore, the via hole and the interconnect groove can be definitely filled with the metal material film. As a result, the electric characteristics of a connection plug filled in the via hole and a metal interconnect filled in the interconnect groove can be improved, and a connection failure between the connection plug and a lower metal interconnect can be avoided. Thus, a multi-level interconnect structure with a good electric characteristic can be formed by the dual damascene method.
The pattern formation method for this invention comprises the steps of forming an organic film on a substrate; forming, on the organic film, a mask layer including an inorganic component; and forming an organic film pattern from the organic film by selectively etching the organic film by using the mask layer and by using plasma generated from an etching gas containing, as a principal constituent, a compound including carbon, hydrogen and nitrogen.
In the present pattern formation method, an opening is formed in the organic film by the present method for etching an organic film. Therefore, an opening with a vertical or forward taper cross-section can be formed in the organic film, and the opening of the organic film pattern can be prevented from having a bowing cross-section. As a result, the etching can be precisely conducted with a large process margin.
In the pattern formation method, the compound is preferably methylamine, dimethylamine, trimethylamine, ethylamine or propylamine.
In this manner, the opening formed in the organic film can definitely attain a vertical or forward taper cross-section.
In the pattern formation method, the mask layer is preferably a silylated layer.
In this manner, an opening with a vertical or forward taper cross-section can be formed in an organic film pattern by a top surface imaging process.